1. Field of the Invention
The invention relates in general to microphones. In particular the invention relates to microphones in mobile phones and their accessories.
2. Brief Description of Related Developments
There is a general need to protect microphones against radio frequency (RF) disturbances for ensuring the proper performance of the microphones. Microphone of mobile phone or mobile phone accessory should furthermore be immune to RF disturbances at the frequencies the cellular system uses. Consider for example a headset accessory of a mobile phone. It has a small earpiece connected to the mobile phone with a wire and the microphone of the headset mounted on a stiff wire at suitable distance from the earpiece so that the microphone can pick up the voice of the user. The user may carry the mobile phone in a pocket during a call. If the user carries the mobile phone in a breast pocket of a shirt or a jacket, the microphone of a headset is very near the radio transmitter of the mobile phone. If the microphone is not adequately protected, it can demodulate the radio frequency signal, in which case the quality of audio signal may deteriorate. Furthermore, there is need to protect microphones against electrostatic discharge (ESD).
Electret microphone is general type of microphones used in mobile phones and accessories. An electret microphone contains a preamplifier, typically a field effect transistor (FET), and voice is converted to electrical signal by capacitance. Changes in the air pressure cause changes to the capacitance between a conductive plate and a conductive polarized foil. The conductive plate, the conductive foil, FET and other microphone parts, which are typically capacitors, are typically placed in side a microphone capsule. This capsule has typically two output contacts with which it is connected to external circuitry.
Electret microphones are protected against RF disturbances and ESD in various ways. FIG. 1 shows an example of a prior-art protection circuit. The circuit relates to a headset accessory of a mobile phone. In the Figure there can be seen a microphone capsule 100, first protection circuit 110, a transmission line 120 between the headset accessory and mobile device, second protection circuit 130 and microphone amplifier 140. The microphone capsule contains an electret microphone M1 comprising FET Q1, which functions as preamplifier. The drain of FET is connected to first output contact OC1 of microphone capsule and the drain of FET is connected to second output contact OC2 of microphone capsule. Further the microphone capsule contains for RF-protection a capacitor C11 connected between the output contacts of microphone capsule. The capacitance of capacitor C11 is small. Together with the stray inductance of capacitor it causes a series resonance at certain frequency band, which is arranged to including the transmitting band of the mobile phone in question. Then the parallel capacitor C11 attenuates disturbances occurring at said band. The problem here is that the RF protection works only at narrow frequency band. The layout of the microphone components on a circuit board inside capsule has to be done very carefully, and changes in the capacitance values of the capacitors, even changes within production tolerances, may cause the RF protection to shift out from the desired frequency band.
The first protection circuit 110 is connected to first and second output contacts of microphone capsul. The circuit includes in succession from the microphone capsule a series coil L11 at second output contact, a parallel ESD protector VDR1, a series coil L12 at first output contact and a parallel capacitor C12. The capacitor C12 and coil L12 are for filtering disturbances. The ESD protector is in this example a voltage dependent resistor (VDR) or varistor. Its resistance drops shorting the circuit when a electrostatic disturbance having relatively high energy arrives along the transmission line 120. The disadvantage of the external varistor is that it has some internal capacitance, which couples with the capacitance of capacitor C11 causing a new resonance. This may lead to RF immunity failures at some frequency band. For this reason there is coil L11, e.g. a ferrite bead, in the protection circuit 110. It weakens said capacitive coupling and corresponding resonance. However the inductance of coil L11 may cause significant resonance at certain other frequencies. It is possible to add a resistor R11, instead of a coil, in series to one output conductor of the microphone capsule, to weaken said capacitive coupling. However such a resistor should be very large to sustain an ESD pulse. Small surface mounted resistors change their resistance and typically fail in ESD tests. Further adding a resistor between the ESD protector and the microphone may cause the microphone more susceptible to ESD.
The second protection circuit 130 at the other end of headset cable is for protecting the actual microphone amplifier 140. The second protection circuit includes a series coil L13 and a parallel circuit forming of a capacitor C13 and a resistor R12 connected in series.
So in conventional design there may be several, up to ten additional components whose purpose is to protect the microphone from ESD and RF disturbances. The immunity to both ESD and RF disturbances is still inadequate. By means of additional components are overcame some problems, but at the same time arise new difficulties. The more components there are in the circuit, the larger are the conductive loops of circuit and correspondingly the greater the susceptibility to RF-disturbances. Further additional components make new resonance problems.